1. Field of the Invention
The present invention relates to a laser apparatus, and in particular to a laser apparatus that uses a plurality of laser diode modules to provide a light emitting source or a pumping light source.
2. Description of the Related Art
Many high-power laser apparatuses are equipped with a plurality of laser light source units in order to achieve high output power, and a variety of driving methods have been reported in the conventional art (for example, refer to Japanese Patent No. 5729107, hereinafter referred to as “patent document 1”, Japanese Patent No. 4341600, hereinafter referred to as “patent document 2”, and Japanese Unexamined Patent Publication No. 2012-124304, hereinafter referred to as “patent document 3”).
Patent document 1 discloses a “laser oscillator control apparatus for controlling a laser oscillator constructed from a plurality of oscillator modules, each adapted to be driven to oscillate a laser beam”. The laser oscillator is configured to produce an output power by collecting the laser beams oscillated by the plurality of oscillator modules, wherein the apparatus includes a module selecting means for selecting the oscillator modules to be driven. The module selecting means includes a selection criterion identifying unit which reduces the number of oscillator modules to be driven, by deciding not to drive all the oscillator modules when the value to be output power per oscillator module, calculated by dividing the output power of a laser beam machine required for laser processing by the number of oscillator modules, is smaller than the minimum controllable output power of each individual oscillator module. Patent document 1 also discloses a “laser oscillator control apparatus wherein when the output of the laser beam machine required for laser processing is not larger than the maximum rated output power of each individual oscillator module, the selection criterion identifying unit reduces the number of oscillator modules to be driven to one by deciding not to drive all of the oscillator modules.”
When the oscillator module light emitting source is a laser diode module, the optical output power versus the driving current injected into the laser diode module has a characteristic such as shown in FIG. 1, and the voltage applied to the laser diode module versus the driving current has a characteristic such as shown in FIG. 2. As a result, electrical to optical conversion efficiency exhibits a peak at a given driving current, as shown in FIG. 3, and thus the electrical to optical conversion efficiency has a shape asymmetrical with respect to that given current. Accordingly, with the selection criterion described in patent document 1, it is not always possible to drive the laser diode module under conditions where the electrical to optical conversion efficiency is highest. Suppose, for example, that there are a total of four oscillator modules, each having an optical output power characteristic, current-voltage characteristic, and electrical to optical conversion efficiency characteristic, such as shown in FIGS. 1 to 3, respectively, and having a maximum rated output of 250 W and a minimum controllable output of 60 W. In this case, when the optical output power command value is 260 W, the condition for reducing the number of oscillator modules to be driven is not yet met in patent document 1. As a result, all the four oscillator modules are driven, each with an output of 65 W. However, as can be seen from FIG. 4, it is clear that the electrical to optical conversion efficiency is higher when only two modules are driven, each with an output of 130 W.
On the other hand, when the optical output power command value is 240 W, since the command value is less than the maximum rated output power of each individual oscillator module, only one oscillator module should be driven according to the latter selection criterion that defines that “when the output of the laser beam machine required for laser processing is not larger than the maximum rated output of each individual oscillator module, the selection criterion identifying unit reduces the number of oscillator modules to be driven to one by deciding not to drive all the oscillator modules.” However, as can be seen from FIG. 5, it is clear that the electrical to optical conversion efficiency is higher when two modules are driven. FIG. 4 in patent document 1 shows examples of calculations performed by the laser module selecting means. For example, case 1 assumes four laser modules each having a maximum output of 500 W. It is described that when the output command value from the processing condition is 1000 W, two laser modules are selected and driven, each with an output of 500 W. However, in the case of laser modules having optical output power characteristics such as shown in FIG. 6, it is clear that the electrical to optical conversion efficiency is higher when all the four modules are selected and driven, each with an output of 250 W. That is, according to patent document 1, when the oscillator module light emitting sources are laser diode modules, there is no guarantee that the modules can always be driven with the highest electrical to optical conversion efficiency.
Further, in patent document 1, it is described that “still another object of the present invention is to reduce the number of oscillator modules to be selected only when effective and thereby to prevent durability from decreasing due to concentrated use of a limited number of oscillator modules.” That is, by avoiding driving a limited number of oscillator modules wherever possible and thereby avoiding applying loads to the limited number of oscillator modules, the life of such oscillator modules, and hence the life of the laser apparatus as a whole, is prevented from being reduced. However, no essential solution is presented to address the problem that the life becomes shorter due to concentrated use of a limited number of oscillator modules. As a result, depending on the processing conditions, a limited number of oscillator modules may continue to be used in a concentrated manner, eventually resulting in the problem that the life of such oscillator modules comes to an end earlier than the life of the other oscillator modules.
Patent document 2 discloses a “solid-state laser apparatus comprising a current distributing means for determining the distribution of current to a pumping light source for each pumping module so that the output measured by an output measuring means becomes maximum under the condition that the total sum of the currents to the pumping light sources of the pumping modules is maintained constant.” This driving condition can be interpreted as distributing the currents so that the total sum of the currents to the pumping light sources becomes minimum, in order to provide a prescribed output. Consider the case where each light source is a laser diode module and each oscillator module has an optical output power characteristic, current-voltage characteristic, and electrical to optical conversion efficiency characteristic, such as shown in FIGS. 1 to 3, respectively, and a maximum rated output of 250 W. Suppose that the number of oscillator modules is four; then, when the optical output power command value is 400 W, the total sum of the currents is smaller when two oscillator modules are driven, but the electrical to optical conversion efficiency is higher when three oscillator modules are driven, as shown in FIG. 7. That is, according to patent document 2, when the oscillator module light emitting sources are laser diode modules, there is no guarantee that the modules can always be driven with the highest electrical to optical conversion efficiency.
Patent document 3 discloses in claim 5 a “method for controlling a high-power two-dimensional surface-emitting laser array, wherein in light source units each comprising one or a plurality of independently controllable high-power two-dimensional surface-emitting laser array elements, the number of light source units to be driven is calculated by dividing the required optical output power by the maximum efficiency optical output power prestored as maximizing the electrical to optical conversion efficiency and by rounding the result, a unit optical output power for each light source unit is calculated by dividing the required optical output power by the number of light source units to be driven, an optical output power difference is calculated between the unit optical output power and the maximum efficiency optical output power, a corrective driving current corresponding to the optical output power difference is calculated from the linear relationship that the optical output power has with respect to a variation in the driving current of the high-power two-dimensional surface-emitting laser array, and the surface-emitting laser array is driven with a driving current obtained by correcting the maximum efficiency optical output power by the corrective driving current.” “Dividing the required optical output power by the maximum efficiency optical output power that maximizes the electrical to optical conversion efficiency and rounding the result” in the above description is described in mode for carrying out the invention that “as a method of rounding the value, the value should simply be rounded to the nearest integer.” By taking the above driving condition into account, the following case is considered: each light source is a laser diode module; each oscillator module has an optical output power characteristic, current-voltage characteristic, and electrical to optical conversion efficiency characteristic, such as shown in FIGS. 1 to 3, respectively, and has a maximum rated output of 250 W; the maximum efficiency optical light output power that maximizes the electrical to optical conversion efficiency is 125 W; and the number of oscillator modules is four. In this case, when the optical output power command value is 185 W, the number of light source units to be driven is calculated as 185 W÷125 W=1.48≈1, and the laser array is driven with a driving current of 8.79 A obtained by correcting by the corrective driving current. However, as shown in FIG. 8, the electrical to optical conversion efficiency is higher when two oscillator modules are driven to produce a total output of 185 W. That is, according to patent document 3, when the oscillator module light emitting sources are laser diode modules, there is no guarantee that the modules can always be driven with the highest electrical to optical conversion efficiency. Furthermore, in patent document 3, the corrective driving current is obtained by using the linear relationship that the optical output power has with respect to a variation in the driving current. However, in the case of high-power laser diode modules, the modules are often driven within a high optical output power diving current region where the optical output power does not vary linearly with the driving current. As a result, when the corrective driving current is obtained using the linear relationship, there arises the problem that the optical output power is not produced with the required accuracy in response to the optical output power command value.